The curing agent for epoxy resin and the coating composition according to the present invention can be used with no specific limitation in various fields and applications, but in the following, the background art relating to “anti-corrosion coatings” is mainly described for convenience of explanation.
Steel constructions such as typically ships, bridges, tanks and plants are exposed to severe corrosion environments over a long period of time, and in general therefor, anti-corrosion coatings are applied thereto.
Such coatings are specifically referred to as heavy anticorrosion coatings. The heavy-anticorrosion coating generally comprises a multi-layered structure as recoated or overcoated in a process of three steps of primer, midcoat and topcoat, in which an epoxy resin-based coating composition is much used for the undercoating and intermediate-coating. This is because an epoxy resin cured product has good adhesiveness to substrates and, in addition, it has a basic property of good anticorrosion. On the other hand epoxy resin based coating compositions are defective in their weather resistance in exposure environments, and in general, overcoating compositions of good weather resistance such as those with polyacylic resin, or fluoro resin are overcoated thereon.
For such heavy-anticorrosion coatings, in general, the epoxy resin coating composition must be recoated as intermediate coating after primer coating and as top-coating after the mid-coating within a predetermined period of time. This is because of the property of ordinary epoxy resin coating compositions in that the intercoat adhesion of the composition may be extremely lower after a certain period of time after coating.
The reasons of the property are described. It is well known that if one waits too long before overcoating, poor intercoat adhesion results. In that manner, in the case where recoating is applied to coated substrates after kept in exposure environments over a predetermined period of time, then so-called sand blasting or water-jet cleaning are needed for removing degraded films or debris from surface of the coating layer with an abrasive. However, when the coated substrates are super steel constructions, then the treatment is accompanied by technical and economical difficulties.
Heretofore, in the field of coating compositions that are referred to as heavy-anticorrosion coating compositions, a coating composition has been employed, in which an amide compound produced from a polymerized fatty acid and a polyethylene-polyamine or its adduct with a glycidyl ether compound added thereto is used as a curing agent and this is combined with an epoxy resin, preferably a solid epoxy resin having a relatively large molecular weight. However, when the substrates are kept in exposure environments for a long period of the time after undercoated, then there may occur some difficult problems in point of the recoatability.
A curing agent prepared by previously reacting a polyamide curing agent and a glycidyl ether compound is referred to as a polyamide-epoxide adduct, and it has been proven in this technical field; however, it is also defective in the resistance to long term exposure, and therefore it has some problems in point of the recoatability thereof with an overcoating composition.
Further, a curing agent for epoxy resin, which is prepared by reacting a phenolic compound such as phenol, cresol, butylphenol, nonylphenol or cardanol, with a polyamine and formaldehyde, has an excellent characteristic of rapid curability in a low temperature range, and has been proven in the field of heavy-anti-corrosion coating compositions. However, it is also not resistant to long-term exposure, and therefore it is also defective in that its recoatability with an overcoating composition.
Recently, in JP-A 2002-114835 (Patent reference 1), an amide-type curing agent for epoxy resin has been proposed, which is obtained by adding a glycidyl ether compound to amide compound produced from a polyamine compound that contains at least 50% of metaxylendiamine and a polymerized fatty acid. The curing agent may realize a good anticorrosive property in some degree but is still insufficient with regard to its recoatability with an overcoating composition.
An objective of the present invention is to provide a curing agent for epoxy resin and a coating composition comprising it, which may solve the problem in the prior art.
Another object of the present invention is to provide a curing agent for epoxy resin and coating competition comprising it, which basically have an excellent anticorrosive property and have good recoatability with overcoating composition after long term exposure.